Transducers



March 14, 1961 J E L 2,975,243

TRANSDUCERS Filed Jan. 17, 1958 2 Sheets-Sheet 1 INVENTOR.

19L BERT I KflTELZ/i F. UT H March 14, 1961 A. J. KATELLA TRANSDUCERS 2 Sheets-Sheet 2 Filed Jan. 17, 1958 INVENTOR. HZEERT J. KHTELLH BY F oi r United States 2,975,243 TRANSDUCERS Filed Jan. 17, 1958, Ser. No. 709,680 8 Claims. (Cl. 179-111) This invention relates to electro-acoustic transducers and mainly to loud-speakers of the electrostatic or condenser type, employing a tensioned membrane.

The acoustic quality of such speakers is significantly affected by the geometrical arrangement of the tensioned, vibratory membrane, relative to its rigid backing means. For instance it is necessary to avoid irregularities of the backing surface to the greatest extent possible within limits such as those imposed by economy, and for this purpose the backing surface is usually provided by a curved backing plate. It is also necessary to make the spacing between this curved surface and the tensioned, vibratory membrane as small as possible. Still further, the actively vibrating, unsupported membrane areas should be as large as possible, and accordingly the boundaries of such areas, provided by membrane supports, should be spaced from one another as widely as possible. Because of the rise of the curved backing plate, relative to the tensioned membrane, the last-mentioned requirement has been hard to reconcile with that of minute and yet uniform membrane-back plate spacing. Other conidiots and diihculties have also been encountered in the attempt to develop a membrane-back plate geometry which meets all acoustic requirements.

It is a primary object of this invention to reconcile the aforementioned requirements in improved ways.

This has been found to be possible by using novel geometry and design as to membrane arrangement. The innovation consists, according to one aspect of the invention, in using membrane spacers or supports which extend spirally or obliquely over curved backing means. It might be expected that such an arrangement would aggravate former problems. The discovery of its useful effect has, however, been substantiated by actual use and in a variety of tests, applied to transducers constructed in accordance with the invention.

The effects of ths feature, in conjunction with associated design characteristics of the transducer, will now be explained in connection with the description of certain preferred embodiments of the invention. Such a description follows, with reference to the drawing, wherein:

Figure l is a perspective front view of one type of apparatus constructed in accordance with the present invention, parts being cut away in the interest of illustration. Figure 2 is an elevational rear view of the same apparatus, on a smaller scale. Figure 3 is an enlarged sectional view taken along a short portion of line 3-3 in Figure 1. Figure 4 is an additionally enlarged detail from Figure 3. Figure 5 is a plan view of an element from Figures 1 and 2.

Figure 6 is a fragmentary view generally similar to the view of Figure 1 but showing a diflerent type of apparatus incorporating the present invention. Figures 7 and 8 are enlarged sectional views, taken along lines 7-7 and 8-8 in Figure 6.

Referring first to Figures 1 and 2: the preferred speaker, shown in these figures, comprises a vibratory membrane 10 interposed between outer and inner membrane spacers 11 and 12, which in turn are interposed between atent Q outer and inner back plates 13, 14. Each back plate has a major portion thereof curved in approximately cylindrical form, and each may have minor portions or tabs 15, 16, 17'bent away from the generally cylindrical surface. The drawing shows the inner backing plate 14 complete, whereas the successive, outwardly superimposed members 12, 111, Hand 13 are progressively broken 011 in Figure l to disclose their sequence and arrangement. Elements 10 to 14, all facing in the same direction with their concave sides, are nested and held together by fast eners 13. They may be mounted on a supporting structure, not shown, for instance by suitable fasteners engaging tabs 1'7 For electrostatic vibration of membrane 10, a conductive layer 19, provided by this membrane (Figure 4) may be connected with a conductor 20 (Figure 1); and a biasing potential, with push-pull signal potentials superimposed thereupon, may be supplied to the double-ended back plate and membrance structure 13, 19, 14 in a known way, for which purpose additional conductors 21, 22 are connected respectively with tabs 15, 16 of plates 13, 14.

As indicated by Figures 1 to 4, the two rigid backing plates 13, 14 are perforated by relatively numerous and small apertures 23, while according to the invention, membrane spacers 11, 12 have, relatively, very large apertures 24 formed therein which extend obliquely to the curvature of plates 13, 14 (Figure l). Desirably plate apertures 23 are circular holes regularly distributed over the areas of the plates, each hole being somewhat larger in diameter than the plate is thick and their combined areas forming a substantial portion, such as one-third, of the plate area. The aforementioned spacer apertures 24 may be of approximately rectangular design and each of them, as shown, is wide enough to cover several of the plate apertures. Most of the spacer apertures, as shown, are several times longer than wide. Membrane 10 is shown in Figure 4 as being laminated and imperforate and as comprising an outer lamina 25 adjacent the outer spacer 1'1, with conductive coating 19 suitably deposited on this outer lamina. The membrane is shown as additionally comprising an inner lamina 26 adjacent inner spacer 12. This inner lamina may be cemented to said coating; and the spacers in turn may be cemented to the membrane. It is also possible to cement the inner and outer spacers to the inner and outer backing plates respectively.

A representative spacer, with apertures 24 according to the present invention, is clearly and fully shown in Figure 5. The spacer is and it should be understood that both spacers 11, 12 desirably have the same construction, design and dimensions. Each of them may comprise a generally rectangular sheet, which may desirably be formed of flexible, electrically insulating, synthetic plastic material, thicker than a vibratory lamina or pair of laminae 25, 26. The said sheet may comprise a pair of elongated, parallel side members 27, 28 and a pair of parallel end members 29, 30, jointly forming a frame. The sheet may further comprise one or more integral portions 31, extending parallel to and in the middle between ends 29, 31 and/or one or several dividers 32, 33 extending diagonally, for instance from a corner between side and end members 28, 29 to a corner between side and divider members 27, 31. According to the invention, and desirably at right-angles to such dividers 32, 33, a series of diagonal members 34 are provided as parts of sheet .11, extending obliquely relative to the sides and ends of the sheet to form the aforementioned, oblique apertures 24.

As clearly indicated in Figure l, spacer apertures 24 extend obliquely of the curvature of back plates 13 and 14, so that in the assembled speaker, apertures 24 have substantially spiraling directions. As initially indicated,

here designated by number 11 this oblique or spiral arrangement has been found mechanically and acoustically superior to the square or coaxial arrangement of spacer apertures. While the underlying reasons for the advantages of the oblique arrangement are complex and, to some degree are obscure at the present time, the nature of the advantages can be demonstrated readily and convincingly.

In this connection the convexity of the surface of plate 14, adjacent membrane 10, must be considered. While the curvature of the back plates may be other than cylindrical, and while the form of the back plates may differ from the form shown, in many other respects, it is known to be important that the plates be definitely curved about some suitable axis or axes. Thus far it has been assumed that in connection with such curvature it is best to use a spacer arrangement according to which the membrane spacers would have long apertures 24A as shown in broken lines and for explanatory purposes in Figure 1, which apertures would extend in directions parallel to the axis of curvature of the back plates. According to such a conventional arrangement, and in contrast to the arrangement of apertures 24 according to this invention, said apertures 24A would be vertical, not oblique. They might be exactly as wide as the oblique apertures 24 are and might be substantially as long as the average length of the oblique apertures 24.

It has been discovered that superior acoustic response is provided by a new speaker, having the oblique apertures 24 as claimed herein. It has further been found that such superiority is due to the surprising fact that the spacing between the vibratory membrane and the inner back plate 14 thereof becomes more uniform when using the oblique arrangement 24, and not, as would be expected, when use is made of the vertical arrangement of apertures 24A mentioned above.

In this connection, reference may be made to Figure 3, wherein broken line 16A is a straight line from one side to the opposite side of an aperture 24. Line 10A also indicates the position which a single tensioned string or band would occupy, relative to the back plates 13, 14, if such a string were tensioned between the sides of aperture 24, for instance between the points X and Y in Figure 1. It is clear that the body of such a tensioned, straight-lined string or band 10A would be relatively close to the convex surface of the inner back plate 14 and relatively far from the concave surface of the outer back plate 13, if and as the points X and Y, as shown in Figure 3, are centered between said plates.

Due to support derived from the spirally curved walls defining apertures 24, and also because of mutual support between the parts of a tensioned membrane, which forms a three-dimensional body much dilferent from a mere series of tensioned bands or strings, the actual position of membrane 10 in aperture 24, as shown in Figure 3, is not that of straight line 10A; it is curved rather than straight-lined, and the entire body of membrane 10, in aperture 24, is spaced from the two back plates by relatively uniform distances. At the very least, as a result, these distances are much more nearly uniform than those between straight line 10A and the plates; and surprisingly they are also more uniform than they would be in vertical aperture 24A. Thus the difference between the position of lines 10 and that of 10A, in Figure 3, substantially illustrates the difference between the geometry of membrane 10 according to this invention and the geometry of a membrane maintained in a conventional arrangement. This can be demonstrated not only acoustically but also by comparing the electrostatic capacitance between members 10, 13 with that between members 10, 14, in both arrangements. Still another test of the approximation to perfectly smooth curvature of tensioned membrane 10 can be performed optically, with members 11, 13 removed.

This close approximation to perfectly smooth curvature of the membrane, gained in accordance with this invention, is acoustically significant in several respects. It is of assistance in preventing the vibrating membrane from contacting the back plate surfaces, which contacting would cause disturbance both in a direct, acoustic way and due to possible electrical depolarization of the membrane, as will readily be understood by persons skilled in the, art. The improved centering of the membrane, provided by the smooth curvature, also minimizes the tendency toward disturbing harmonics, which tendency is proportional to the square of any eccentricity of the membrane, as explained for instance in co-pending application Serial No. 679,290, filed August 20, 1957, by Lloyd J. Bobb and assigned to the assignee of the present invention. The improved centering also allows the use of desirably small depth of apertures 24 and of desirably large areas of apertures 24; and in turn, the shallowness of said apertures greatly adds to the sensitivity of the transducer, while their Width and area similarly adds to the transducers sound output, mainly at low frequencies. It has long been desired to arrange the individual, vibrating membrane areas so as to provide them with maximum area, minimum distance from the back plates, and maximum uniformity of such spacing, and yet to keep the plate, spacer and membrane construction reasonably simple and inexpensive. This object has been furthered in large measure by the oblique spacer arrangement.

It may also be noted that the new construction can be kept very simple and inexpensive. The identical spacers .11, 12 can be punched out of insulating sheet material without any need for high precision of the punched-out pattern. The punching operation may simultaneously serve to provide suitable holes 35 (Figure 5), for the fasteners 18 (Figures 1 and 2), and'rnay desirably also provide a recess 36 (Figure 5) to leave space for the installation of terminals 37, 38 (Figures 1 and 2), for the connection of conductor 20 to the metallic layer 19 (Figure 4); the precise manner of connection, as by rivets 39 (Figure 1), being a matter of indifference for the present invention. Membrane 10, as mentioned, can be cemented to spacers 11, 12 and thus a sheetformed membrane and spacer unit can be formed, which can simply be rolled onto the convex side of the inner plate 14 and finally be compressed between that plate and the outer plate 13.

Reference will be made now to Figures 6 to 8, which disclose the construction of a single ended electro-acoustic transducer embodying the present invention. This embodiment of the invention uses a single, curved back plate 40, with oblique spacers such as threads 41 thereon and with a membrane 42 tensioned over these spacers. The back plate has numerous, distributed apertures 43, which are small in proportion to the distances between the threads 41. The tensioning of the membrane may be obtained by known means, not shown, for instance as shown in said application of Lloyd J. Bobb, or his prior application Serial No. 636,891, filed January 29, 1957, and assigned to the assignee of the present invention. In this modified embodiment, as well as in the apparatus of Figures 1 to 5, it becomes possible to use an advantageously wide spacing between desirably thin membrane spacers, while minimizing non-uniformity of membrane spacing and avoiding plate-membrane contacts.

While only two embodiments of the invention have been described, it should be understood that the details thereof are not to be construed as limitative of the invention, except insofar as is consistent with the scope of the following claims.

I claim:

1. In an electro-acoustic transducer: a backing structure having a convex curved surface; spacer means curved along and spirally extended across said surface at distances from one another; and vibratory membrane means tensioned on said spacer means over said surface.

2. In a transducer as described in claim 1, apertured sheet means, providing the spirally extended spacer means as integral parts of the sheet means between the apertures thereof.

3. In a sound reproducer: a tensioned membrane; at least one backing member supporting said membrane and having thickness and rigidity suflicient to withstand tension applied thereto by the so supported membrane, said backing member having a curved surface; and a network of membrane spacer elements curved similarly to and extending along said surface, between said membrane and said backing member in predominantly elongate, parallel zones spiralling obliquely over the curvature of the curved surface.

4. A sound reproducer as described in claim 3 wherein the backing member comprises plate means having apertures distributed thereover and the membrane spacer elements extend over said surface at distances from one another which equal multiples of the distances between said apertures.

5. A sound reproducer as described in claim 3, wherein said network of spacer elements comprises an approximately rectangular sheet of flexible material, said sheet having long apertures extending obliquely to the sides of the sheet.

6. A sound reproducer as described in claim 5 wherein said sheet comprises, as an integral part thereof, divider means extending across said oblique apertures.

7. As a membrane and spacer unit for an electro-acous tic transducer: an approximately rectangular sheet having elongate perforations oblique to the sides of the sheet; and a vibratory membrane overlying and secured to said sheet and comprising an electrically conducting layer; said sheet and membrane being adapted to be flexed into arcuate shapes, so as to tension said membrane.

8. A loud-speaker comprising: a pair of rigid, electrically conductive back plates, each perforated by small, distributed apertures and each formed substantially like a segment of a cylinder; a pair of similarly formed, mutally similar membrane spacer sheets coextensive with and interposed between said plates, each having large, distributed apertures, elongate diagonally of the rectangular outlines of the plates; a vibratory, electrically conductive membrane, centered between and smoothly following said curved surface, while tensioned across the apertures of the spacer sheets; means for clamping the membrane between the spacer sheets and plates; and a system of conductors, including one connected to the membrane and one connected to each of the plates.

References Cited in the file of this patent UNITED STATES PATENTS 1,622,039 Lee Mar. 22, 1927 1,644,387 Kyle Oct. 4, 1927 1,811,482 Tayler June 23, 1931 1,975,801 Rieber Oct. 9, 1934 2,824,178 Bobb Feb. 18, 1958 2,863,953 Reed et al Dec. 9, 1958 2,878,323 Bobb Mar. 17, 1959 2,880,260 Strauss Mar. 31, 1959 

